Recent studies suggest that FGF14, a member of the intracellular fibroblast growth factor (iFGF) subfamily functions as a novel regulator of neuronal excitability. The major phenotype in mice lacking Fgf14 (Fgf14-/-) is ataxia and mutations in FGF14 in humans cause a progressive spinocerebellar ataxia syndrome, SCA27. It has also been demonstrated that FGF14, and other iFGFs interact with the C-terminal domains of voltage-gated Na+ (Nav) channel pore-forming (1) subunits and modulate the properties of heterologously expressed Nav channels. In addition, exploiting a validated (in Fgf14-/-mice) anti-FGF14 specific antibody, we find that FGF14 co- localizes with Nav channel 1 subunits at the ankyrin G-rich axon initial segments (AIS) in cerebellar Purkinje neurons, These observations led us to hypothesize that loss of FGF14 produces a defect in the firing properties of Purkinje neurons, the sole output neurons of the cerebellar cortex. In preliminary studies focused on exploring this hypothesis directly, we found that spontaneous activity and repetitive firing were decreased significantly in Fgf14-/-, compared with wild type, Purkinje neurons. Additional preliminary studies revealed that the expression and AIS localization of the Nav channel 1 subunit, Nav1.6, was reduced markedly in Fgf14-/- Purkinje neurons, whereas Ankyrin G expression at the AIS was not significantly affected. These findings suggest that FGF14- Nav 1 subunit interactions play a critical role in regulating the expression and/or the AIS localization of Nav channels and in controlling the firing (output) properties of cerebellar Purkinje neurons. The experiments outlined in this proposal will test these hypotheses directly and explore the molecular mechanisms involved in mediating the effects of FGF14 on the expression, localization and functioning of Nav channels in cerebellar Purkinje neurons. Additional experiments will be focused on testing directly the hypothesis that the SCA27-linked FGF14 mutant protein, FGF14F145S, functions in vivo as a dominant negative to disrupt the interaction between the wild type FGF14 protein and Nav channel 1 subunits, thereby reducing Nav channel expression/localization and altering the firing properties of cerebellar Purkinje neurons. It is anticipated that these studies will provide new and fundamentally important insights into the functional roles of FGF14 and into the underlying molecular mechanisms involved in FGF14- mediated effects on neuronal excitability.